A majority of flat-rolled steel products are produced at least in part in continuous hot strip mills wherein steel slabs are hot rolled to hot-rolled sheet and strip products, either in finished form, or semi-finished form for further processing.
Whether the hot rolled steel is a finished product or semi-finished for further processing (e.g., cold rolling to cold rolled products), the hot rolled steel should be cooled uniformly at a controlled rate for the purpose of achieving a predetermined microstructure and accordingly certain physical properties in the hot rolled product before the steel reaches the coiling stand. Specifically, the cooling rate must not be so fast as to quench the steel to a brittle condition, but must be fast enough to achieve transformation to a predetermined ferrite microstructure depending upon the properties desired. Also, the cooling rate should be sufficiently uniform such that the entire steel coil has reasonably uniform physical properties at all locations along its length and across its width.
Typically, a number of water spray nozzles are horizontally positioned for the purpose of spraying water onto the workpiece as it moves along a runout table towards a coiling stand. Prior art cooling systems have traditionally comprised a plurality of horizontally positioned, side-by-side water pipes which extended transversely above the path of travel of the workpiece, with each water pipe provided with a number of spray nozzles to spray the workpiece as it moves along the runout table. Considerable improvements have been made in the apparatus in recent years. Specifically, more modern cooling systems typically provide headers or water boxes positioned transversely above the runout table, each provided with nozzles to direct a low-pressure stream of water onto the moving workpiece. The headers are normally provided with a plurality of siphon tubes or elongated "stand-pipe" type nozzles which direct a low-pressure laminated flow of cooling water downwardly onto the workpiece from above. Such low-pressure laminar flows of cooling water have been found to provide an increased and more uniform cooling rate than the prior spray nozzles which virtually spray the water onto the hot steel, and minimize splashing to achieve a more uniform cooling of the workpiece.
It is also important that the cooling system utilized be suited for cooling moving workpieces of differing thicknesses and widths and workpieces moving at differing line speeds, and even suitable to provide uniform cooling rates even if the workpiece strip speed is accelerated or decelerated while moving on the runout table. All of these requirements necessitate rather complicated cooling systems, which, pursuant to current practices, are provided with valve means for selectively turning-on or turning-off the water to the various headers or header portions thereby switching certain headers or portions thereof into or out of operation in response to need for varying the overall cooling rates or varying the width of the water spray to accommodate workpieces of differing widths. When such changes are made, the response time should be rather quick and certain without any delays, and without any non-uniform commencement or cessation of the water spray distribution. The syphon tubes and stand-pipe tubes now commonly utilized for nozzles are designed to draw water from the upper surface of the header, and do, therefore, provide the advantage that shut-off and turn-on of the water supply to the header will normally provide a very quick response in the stoppage or commencement of water flow through the elongated nozzles.
When water is applied to a metal strip at an even cooling flow rate across the width of the hot strip, there is a flow of water from the center towards the edges of the strip which causes cooling of the edge faster than the middle portion of the strip. Solving of this problem, at present, is attempted by either shaping of a slot through which the cooling water is discharged onto the strip, by redistributing water across the strip width, or by partial deflection of water by the use of solid deflectors. Such systems are, however, of complicated design and in the form of cumbersome equipment, which do not provide a smooth transition between chilled and non-chilled areas of the strip.
It is an object of the present invention to provide a cooling system for cooling of a hot metal strip which uses uncomplex and lightweight equipment to control the flow of cooling water across the width, or portion of the width, of hot metal strips of various widths and which provides for a quick and smooth adjustment of rates of cooling across the metal strip.